In America, the excitement about Dick Cheney's shooting accident is over. There are no more talkshow debates about why he took so long to make a statement, and no more news reports about his 78-year-old victim. Even the delicious contrast between the vicepresident's bravery in the face of small birds and the deferments he took to keep from going to Vietnam no longer raises eyebrows. Yet the shrewdest comment I heard on the incident was rarely touched on. What did the vice-president think he was doing, inquired a serious hunter? Real men got up early and went into the countryside hunting wild quail alone with their dog. Going in groups to a farm to shoot specially bred birds was for sissies. It wasn't Cheney's involvement in masculine pursuits that was noteworthy; it was that the mode of masculinity on show was bogus.

Bogus masculine posturing seems to be the style of the current US administration. Its most conspicuous expression was perhaps Bush's "Mission Accomplished" photo opportunity after the invasion of Iraq. There he was, this veteran of the home guard, clad in a snug-fitting flight suit, strutting the deck of the USS Abraham Lincoln among real warriors, and claiming victory. It was, wrote one commentator, "a masculine drag performance". Similar posturing went on in the Republican convention before the last presidential election: politicians whose own warlike masculinity was nonexistent strove very effectively to effeminise John Kerry, who really had been a hero. So we had Cheney, rather obscenely, accusing the Democratic candidate of wanting to show al-Qaida a "softer side", and muscle-bound Arnold Schwarzenegger making his famous reference to "girlie-men".

Why do current US political officeholders feel the need for such a transparent strategy, and why does it seem to work? To be sure, political power and shows of masculinity have traditionally gone closely together. In the past, rulers led their troops into battle and, even in peacetime, called themselves fathers of their people. And modern politics retains abundant masculine rituals. Prime minister's question time in Britain, for instance, is a stylised duel and tournament redolent of testosterone. By way of voice lessons, wearing severe suits and her own aggression, Margaret Thatcher mastered it (the verb seems appropriate).

Yet the historic fact that power has usually been male scarcely explains why American politicians now appear to feel an obligation to try so very hard. Nor does it explain why Kerry's Purple Heart and Silver Star, won in combat, didn't win greater electoral dividends. As far as the latter's failure with the voters was concerned, I suspect that his allusions to his own heroism in the Democratic convention ("reporting for duty") struck a false note. Anyone who has spoken to experienced combat troops knows that they rarely brag about their exploits. Strong and silent is the preferred style.

The fact that Kerry was encouraged by his advisers to deviate from this mode, rather than maintaining a dignified reticence about his Vietnam record and letting it speak for itself, was yet another aspect of the Democrats' ineptitude in the last presidential election. None the less, the tendency of some US voters to dismiss Kerry, despite all his medals, as "French" - which for Americans, as for Britons, is often a euphemism for effeminate - and to be impressed by George Bush's bluster, his wearing of a Stetson, a leather jacket and cowboy boots on his ranch, and images of him chain-sawing trees, suggests at the very least a degree of confusion about what does constitute masculinity.

This is surely one reason why the Republicans - and, indeed, some Democrats (think of Bill Clinton's busy sexual adventurism) - have been tempted in recent times to use postures of masculinity to such a crass degree. They are not acting this way because Americans possess a strong and confident cult of the masculine virtues, but rather because many are anxiously uncertain about just what these virtues are. These uncertainties stem in part from America's own domestic situation. In some respects, female emancipation has progressed further there than in Europe. At present both the Republican and Democratic parties possess powerful female figures who may well come into play in 2008, in Condoleezza Rice and Hillary Clinton. By contrast, in Britain female MPs were nowhere in the recent Conservative and Lib Dem leadership contests - just as they will be nowhere in the race to lead the Labour party when Tony Blair stands down.

Partly because women in the US are better represented in the hierarchies, the culture wars over gender there have been particularly fierce. This can be seen in the ferocity of the debates over gay marriage, but also over far less serious things. It should, for instance, have come as no surprise that Brokeback Mountain, with its deconstruction of one of the most iconic American male heroes - the cowboy - did rather better at the Baftas in London than at the Oscars in Hollywood. For some Americans, I suspect, this movie was too uncomfortable, even heretical. It scratched at issues that were already irritating.

One way of understanding the bogus masculine posturing of the likes of Bush and Cheney is to view it as a kind of comfort blanket being knowingly extended to troubled American voters (of both sexes) who feel deeply worried that conventional gender roles in their country are unravelling. Male blue-collar workers, who have witnessed the disappearance in recent years of large numbers of conventional masculine jobs in heavy industry, and evangelical Christians concerned about the sanctity and survival of the family are particularly susceptible to such strategies on the part of knowing politicians, however crude and artificial they may seem to non-believers.

There is, however, another factor in play. In Britain, as in the rest of Europe, politics remains overwhelmingly a male pursuit, but it is no longer necessary to try too hard. David Cameron could get away with simply patting his pregnant wife's bump, a New Man gesture that was also, of course, a gesture of proprietorship and potency. But Blair and he do not need to strut upon battleships, however much they might enjoy doing so. Britain, like other European states, is not and never will again be in the topworld- power league, so its male leaders can afford to play subtler, more variegated roles. Leaders of the US don't have that option. They preside over an empire, over the biggest military power the world has ever seen, which is now at war. The pressure on them to be seen to be conventionally masculine is therefore enormous. Just how Hillary Clinton in particular will cope with this in 2008 is not clear.

· Linda Colley is the author of Captives: Britain, Empire and the World, and professor of history at Princeton University lcolley@princeton.edu